Mastering MathCad Version 7 by Keller and Crandall.
and
MathCad a Tool for Engineering Problem Solving by Phil Pricthard.
Of the two, I recommend the Keller and Crandall text as it explores more of what the program can do with extensive examples and programming examples. The other text, despite the title, is a very basic introduction and was really a waste of money for me.